Warm-up:
At the piano, play notes and sing them (with their name.) Make sure you have a high rate of redundancy in your note choices. If practicing in a group, make sure each note lets long enough for everyone to match pitch.

Play single notes for 5-10 minutes, then play pairs, then threes. If playing (and remembering) three note combinations become easy, begin practicing four (or more) note combinations.

Group sing 1: Drone.

Sing a long tone, "merge" voices to a common pitch and hold it. Then, one by one, sing a solo above the drone.

Group Play 1: Arrange the group in a circle. begin with a subset of the group improvising together. Then, going clock-wise, a new member will begin improvising, while one member of the group will "retire", going silent. work your way along the circle several times. As each new member joins in, they should attempt to shift the tone of the music to a new direction, while the members of the group already playing attempt to match sim/her.